Municipal solid waste incineration (MSWI) has been widely used due to its benefits in reducing waste and recovering energy. However, MSWI fly ash and bottom ash are increasing rapidly, causing harm to human health and the environment. This paper discussed the production process, physical and chemical properties, leaching properties, pretreatment methods, and applications of fly ash and bottom ash. By summarizing the previous literature, it is found that MSWI fly ash and bottom ash have mechanical properties similar to natural aggregate. Many beneficial attempts have been made in cement concrete aggregates, ceramic raw materials, and highway engineering materials. Due to concerns about the leaching of heavy metals in fly ash, its application in highway engineering is limited. The application of bottom ash in asphalt pavement is rare because of the side effect on the performance of asphalt mixture. Considering the solidification effect of cement on heavy metals and the low cost of fly ash and bottom ash, the application in cement-stabilized macadam base has broad application prospects. This is beneficial to reduce the construction cost and promote the process of waste incineration, especially in developing countries.

/pdf/physical-and-chemical-properties-pretreatment-and-recycling-19vjjq1aru.pdf

Physical and Chemical Properties, Pretreatment, and Recycling of Municipal Solid Waste Incineration Fly Ash and Bottom Ash for Highway Engineering: A Literature Review

The anthropogenic toxic metal dissemination in surface water resources is a major ecological and public health concern at diverse scales, from the regional to the global scale. However, in the scientific literature, a review dealing with distributions, pollution levels, potential sources and related health risks of toxic metals in surface water resources at the continental, country and global levels are missed. Conducting such a study will shed light on how the most recent investigations reveal that the average concentrations of iron (Fe), lead (Pb), chromium (Cr) and mercury (Hg) in Asia; Fe, Pb, Hg, Cr, cobalt (Co), nickel (Ni), cadmium (Cd), and arsenic (As) in Africa; As, Hg and Cr in Europe, and North, Central and South America are exceeding the drinking water permissible limits set by the World Health Organization (WHO), respectively. To achieve this goal, we analysed datasets belonging to 233 articles published from 2010 to 2022. After conducting a multivariate statistical analysis, our meta-analysis demonstrated both anthropogenic and natural sources were responsible for the high toxic metal concentrations. Moreover, health risk assessment findings showed that the hazard quotient (HQ) values of As and Pb for the ingestion pathway, and HQ value of Pb for the dermal pathway were greater than one (HQ>1) for all continents, thus leading to non-cancer health risks. The novelty of our findings suggests that necessary control measures and robust policies should be implemented urgently to prevent an increasing contamination load in surface water bodies, save natural ecosystem areas and reduce potential human health risks, not only at the regional or national level but also at the global scale. 

A global meta-analysis of toxic metals in continental surface water bodies

This study conducted a review on the concentrations, spatial distribution and pollution assessment of metals including As, Hg, Cd, Co, Cr, Cu, Mn, Ni, Pb and Zn in 102 river sediments in China between January 2008 and July 2020 based on the online literature. The geo-accumulation index (Igeo) and potential ecological risk index (RI) were used for the pollution assessment of the metals. The results showed that the ranges of metals were: 0.44 to 250.73 mg/kg for As, 0.02 to 8.67 mg/kg for Hg, 0.06 to 40 mg/kg for Cd, 0.81 to 251.58 mg/kg for Co, 4.69 to 460 mg/kg for Cr, 2.13 to 520.42 mg/kg for Cu, 39.76 to 1884 mg/kg for Mn, 1.91 to 203.11 mg/kg for Ni, 1.44 to 1434.25 mg/kg for Pb and 12.76 to 1737.35 mg/kg for Zn, respectively. The median values of these metals were descending in the order: Mn > Zn > Cr > Cu > Pb > Ni > Co > As > Cd > Hg. Compared with the SQGs, As and Cr manifested higher exceeding sites among the metals. Metals of river sediments manifested a significant spatial variation among different regions, which might be attributed to the natural weathering and anthropogenic activity. The mean Igeo values of the metals presented the decreasing trends in the order: Cd > Hg > Zn > Cu > As > Pb > Ni > Co > Cr > Mn. Cd and Hg manifested higher proportions of contaminated sites and contributed most to the RI, which should be listed as priority control of pollutants. Southwest River Basin, Liaohe River Basin, and Huaihe River Basin manifested higher ecological risks than other basins. The study could provide a comprehensive understanding of metals pollution in river sediments in China, and a reference of the control of pollutant discharge in the river basins for the management.

Concentrations, Distribution, and Pollution Assessment of Metals in River Sediments in China.

Stabilization of Waste-to-Energy (WTE) fly ash for disposal in landfills or use as cement substitute.

In this study, the long-term leaching behaviors of Cd, Cr, Cu, Ni, Pb, and Zn in municipal solid waste incineration (MSWI) fly ash samples from grate-type (GT) and circulating fluidized bed (CFB) incinerators were investigated and compared under the simulated landfill leachate corrosion scenario, which was determined to be more severe than the acid rain corrosion scenario. The total heavy metal contents showed increasing hierarchies of Ni<Cr<Cd<Cu<Pb<Zn in the GT fly ash samples and Cd<Ni<Cr<Pb<Cu<Zn in the CFB fly ash samples. During the leaching processes, all heavy metals followed the two-stage leaching mode, including quick accumulation in stage 1 and then stable release in stage 2. The heavy metals with the highest accumulative leaching amounts were Cd, Pb, and Zn in GT fly ash and Cr, Cu, and Ni in CFB fly ash. In the landfill leachate corrosion scenario, Cd and Cr showed cationic patterns while Pb, Zn, and Cu showed amphoteric patterns. The leaching of Cd, Ni, and Cr arose from the dissolution of the salts they formed (solubility control), while the leaching of Cu, Pb, and Zn was controlled by the Ca-bearing compounds (sorption and precipitation control). A large difference in Pb leaching was observed: the cumulative leaching amount of GT fly ash (707.59–3072.36 mg/kg) was an order of magnitude higher than that of CFB fly ash (22.47–407.314 mg/kg), as a result of the higher primary content and larger proportion of the residual fraction in CFB fly ash. The acid-soluble and reducible fractions exhibited higher percentages than those of other fractions representing higher levels of environmental toxicity and risk. Therefore, more emphasis should be placed on the conversion of bioavailable fractions into stable fractions for the stabilization and utilization of MSWI fly ash.

Comparison of MSWI fly ash from grate-type and circulating fluidized bed incinerators under landfill leachate corrosion scenarios: the long-term leaching behavior and speciation of heavy metals

Distribution and fraction of heavy metals (Pb, Cr, Zn, and Mn) in water and sediment in Liangtan River, Chongqing was analyzed using the atomic absorption spectrometry method. Pearson correlation coefficient, bioavailability index, geoaccumulation index and potential ecological risk index were introduced to classify the source and assess the environmental risk of the heavy metal. Pb, Cr, and Zn mainly existed in residual fraction and fraction bounded to Fe–Mn oxides while Mn mainly existed in the fraction bounded to carbonates and the residual fraction. Mn and Pb, Zn and Cr in the sediment were confirmed originating from, separate common sources. Igeo and were both in the order of Pb > Zn > Cr > Mn. Geoaccumulation indexes demonstrated that the Liangtan River has been heavily polluted by Pb and Zn with the class of 2–4 and 2–3, independently. From viewpoint of the whole Liangtan River, the mean RI (45.1) belonged to low risk level.

https://www.tandfonline.com/doi/pdf/10.1080/09542299.2016.1204626

Contamination and comprehensive risk assessment of heavy metals in Liangtan River, Chongqing, China

Municipal solid waste incineration (MSWI) fly ash is by-product and hazardous waste produced from MSWI plant. In the MSWI fly ash there are high contents heavy metals, among which cadmium (Cd) is more active and toxic. Although inorganic acid leaching is an effective way to remove heavy metals out from the MSWI fly ash and nitric acid has great efficiency for Cd removal, little literature reported the redistribution of remaining Cd in the MSWI fly ash. This investigation focused on the change of different factions (exchangeable, bound to carbonates, bound to Fe–Mn oxides, bound to organic matter and residual) of Cd in treated (i.e. washed with nitric acid) MSWI fly ash. Sequence extraction procedures (SEP) have been used to derive different fractions of Cd, results indicated that fractions of Cd have changed significantly after nitric acid washing procedures. Due to the changes of main compounds and microstructures stable Cd (bound to organic matter and residual) had opportunity to leach out, which resulted in a higher potential risk (or higher bioavailability index) for living creatures, although the total amount of Cd decreased. X-ray diffraction (XRD) and images of scanning electron microscope (SEM) proved these changes in washed MSWI fly ash.

Distribution of remaining Cd in MSWI fly ash washed with nitric acid

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Contamination and comprehensive risk assessment of heavy metals in Liangtan River, Chongqing, China

Distribution of remaining Cd in MSWI fly ash washed with nitric acid